Secondary electron emitting system



Jan. 10, 1956 1 .R.Kol l-:R

SECONDARY ELECTRON EMITTING SYSTEM Filed Aug. 8, 1951 Inventor". .Lewis RKoller,

l-ls Attorne.

United States Patent SECONDARY ELECTRON EMITTING SYSTEM Lewis R. Koller, Schenectady, N. Y., assignor to General The present invention comprises an electron discharge device containing a light-transparent electrode the surface of which is electrically conductive and yields with high eiiiciency secondary electrons when bombarded by primary electrons, whereby the utility of apparatus making use of such electron emitter is increased and its functing is improved.

For some types of electronic apparatus, such, for example, as television tubes described and claimed in a copending application of Donald E. Norgaard, Serial Number 219,813, filed April 7, 1951, now Patent No. 2,685,660 (and assigned to the same assignee as the present application), it is desirable that secondary electrons should be emitted from a source which is suiciently transparent to permit an image to be viewed therethrough.

My invention comprises an electrical discharge device containing the combination of a transparent base, ordinarily constituted of glass, and a transparent conducting coating formed thereon which is capable of emitting secondary electrons under excitation by primary electrons.

The accompanying drawing shows in Fig. 1 a television device containing a secondary emitter made in accordance with my invention and Fig. 2 is an enlarged sectional view of part of the electrode structure.

Fig. 1 of the drawing shows in side elevation and partly in section a television receiver tube comprising a conical member 1 and a communicating tubular extension 2. An electron gun of known construction is located in the tubular tube extension. The electron gun comprises an electron-emitting member 3, which is heated to electron-emitting temperature by a. filament 4. The emitted electron beam is controlled by a grid electrode 5. vSuitable current-conveying conductors 6 are sealed into the wall of the extension 2. A focusing electrode or first anode 7 also is provided. The conical tube member 1 which consists of metal functions as a second anode. .Proper deflection and focus of the electron beam is secured by well-known focusing and scanning devices represented by the windings 9 and 10 respectively.

The large end of the receiver tube is closed by a slightly convex clear plate 11, which ordinarily consists of glass, but may consist of other suitable transparent materials, another example being fused quartz. The edges of the plate 11 are fusion-welded to a metal ring 12 which in turn is welded to the metallic side wall of the conical tube member 1. `A structure comprising a transparent electron-receiving and emitting electrode 13 and a perforated conductive plate 14 is supported by metal brackets 15 and 15. The parts 13 and 14 are electrically insulated from one another by suitable insulation 16 as shown, and are attached to the conical member 1.

The electrode 13 embodying my invention functions as a target for primary electrons. As sectionally shown in the conventional enlargement of Fig. 2, this electrode comprises a base 17, consisting of light-transmitting material, the surface of which is provided with an electrically conducting coating 18. T he latter may be condensed on the surface of the glass base from the vapor state, and

ordinarily may be applied by spraying tin chloride on the surface of the glass where it appears to react with absorbed moisture. A coating 19 of magnesium fluoride, or its equivalent, for example, calcium uoride, is superimposed upon the conducting foundation film 18. For this purpose, the fluoride may be volatilzed in vacuo from a resistance heater (not shown) shaped to hold a small amount of the substance to be volatilized. This step ordinarily will precede the mounting of the electrode in the envelope 1.

The uoride coating should have a thickness in the range of about to 300 Angstroms. In this range the yield of secondary electrons which are emitted as a result of primary electron bombardment is substantially independent of the film thickness and is about 8 to 10 fold greater than the number of primary electrons which strike the emitting surface. The primary voltage corresponding to the peak emission increases as the film thickness increases, being about 400 volts at a thickness of 75 A. and 600 volts at 300 A. Films thicker than about 300 A. are characterized by a decreased yield of secondary electrons with respect to impinging primary electrons and show an undesired time lag effect. Films as thick as 275 A. are insulating and yield only one secondary electron per impinging primary electron.

The perforated plate 14 also functions as an electrode as will be hereinafter described. lt is provided with perforations about 5 mils in diameter spaced about 20 mils apart. The perforations are sufficiently numerous to permit about 50% of the primary electrons in the electron beam emanating from the electron gun to be transmitted through the plate 14.

In the described cathode ray tube the center of the electrode structure 13--14 is at about the center of deilection of the coil 10. The cathode 3 may be maintained at an unidirectional potential of 10 kilovolts or more negative with respect to the second anode 1 constituted by the metal wall of the conical portion of the envelope. The conductors 20, 20' are leads to the source of potential. The conducting layer 18 on the plate 17 is connected by a conductor 21 to a source of reverse potential of 9.4 to 9.6 kv. The coating of magnesium fluoride is adapted to give an efficient yield of secondary electrons. The potential between the cathode 3 and the conducting coating 18 is about 0.4 to 0.6 kv., which effects bombardment of the coated electrode 13 by the primary electrons emitted by the cathode 3.

Secondary electrons emitted by the film 19 as a consequence of bombardment by the portion of the primary electron beam traversing the openings in the perforated plate 14 are accelerated by the reversing effect of the negative potential to strike the perforated electrode 14 on the side facing the transparent electrode 18-19. This surface is coated with an appropriate phosphor, as for example, a mixture of zinc sulfide and zinc cadmium sulfide, activated with manganese. A visible image is formed which can be viewed through the window 11. Other suitable phosphor coatings may be employed. As a result of the large ratio of secondary electrons to primary electrons, images of enhanced brightness are produced on the phosphor-coated surface of the electrode 14.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A cathode-ray tube comprising an envelope having a transparent target located adjacent one end wall of said envelope, an electron gun supported within said envelope remote from said target for projecting electrons upon said target, a perforated electrode positioned between said end wall and said gun, said electrode having a luminescent phosphor responsive to electron bombardment which is coated on a surface adjacent said target, a transparent conductive coating on the inner surface of i said target facing said perforated electrode and a transcoating on said screen electrode on the side facing said parent film of material having high secondary electron lm. emission characteristics superimposed on said coating. References Cited in the le of this patent 2. A secondary electron emission system comprising the combination of a transparent supporting plate, Van UNITED STATES PATENTS electrically `conductive transparent coating on a surface 2,198,329 Brul-ling et al, Apr. 23, 1940 of ,said plate, .a transparent lm of material having high 2,204,252 Krenzien June 11, 1940 secondary velectron emission characteristics superimposed 2,250,283 Teal July 22, 1941 on said conductive coating, a perforated screen electrode 2,577,038 Rose Dec. 4, 1951 supported 'closely .adjacent said plate and a phosphor l0 

